In recent years, the growing market of portable devices such as mobile phone, PDA, and digital camera, has increased the demand for nonvolatile memory. Especially FeRAM (Ferroelectric Random Access Memory) is receiving much attention because it requires small power consumption and has high-speed data read/write capability.
FeRAM comprises memory cell created by combining capacitor element using ferroelectric material as insulator and cell selecting transistor, or memory cell composed of FET with capacitor element in its gate electrode part using ferroelectric material, and stores data by utilizing the residual polarization of the ferroelectric material.
FeRAM has a number of desirable characteristics: it can operate at low voltage, has a rewrite frequency greater than flash memory and EEPROM (Electrically Erasable Programmable Read-Only Memory) by multiple digits, and is capable of high integration due to the fact that its circuit structure is similar to that of DRAM (Dynamic Random Access Memory).
However, data stored in FeRAM cannot be read accurately when placed in a high temperature environment (sometimes referred to as thermal stress hereinafter) because the capacity of the capacity element decreases due to the physical characteristics of ferroelectric material. In order to solve this, a temperature detecting circuit that detects the ambient temperature of FeRAM and generates an alarm signal when exceeding a prescribed temperature can be used for detecting performance degradation. As a temperature detecting circuit, for instance, a semiconductor integrated circuit that utilizes the voltage-current characteristics of diode for temperature sensing is described in Patent Document 1.
In Patent Document 1, a constant current Iref is flowed in a bipolar transistor Q1, of which the base and collector are connected, using a constant current circuit 100 as shown in FIG. 3, and an abnormal ambient temperature is detected by observing a voltage change VBE between the base and emitter of the transistor Q1.
Paten Document 1
Japanese Patent Kokai Publication No. JP-A-56-135963